Ticket 1406 cancel calculation

[rjbourne]

This PR adds a cancel button to the generic scattering calculator to cancel computations between each chunk of Q values. If the computation is cancelled the remainder of the Iq values are filled in with NaN. The ticket for this request suggests using calcthread.isquit(), however the current code uses twisted.internet threads as opposed to the calcthread class in sasview. As far as I can tell there is no inbuilt way to detect an interruption in these threads, so a boolean flag is used. Upon cancelling the calculation the compute/cancel button becomes disabled until the current chunk of Q values have been computed.

Currently the areas of the graph with NaN values are simply not updated, which can cause odd effects on the graph when cancelling a calculation. This is because the numpy.nan_to_num function in the current code is not correctly implemented, an issue fixed in #1888.

fixes #1406

[rozyczko]

This looks good and performs as expected.
Using QThreads should probably be preferred in this Qt app, but back when I wrote it, it didn't really behave properly.
It could probably be good to remove twisted and use QThreads throughout.